1. Field of the Invention
This invention relates generally to isotope production in light water power reactors.
2. Description of the Related Art
A wide variety of radioactive isotopes are used for medical, industrial, research, and commercial applications. In general, radioisotopes may be produced by irradiating target isotope materials with nuclear particles. The target atoms either transmute directly into the desired isotope, or a radioisotope is produced through a chain of absorption and decay that subsequently generates the desired radioactive product.
The latter is the case, for example, in the production of the important medical imaging isotope Technetium-99m, which represents about 90% of the isotopes used in today's nuclear medicine. Tc-99m produces high-energy gamma rays, which makes detection easy, and its short half-life of 6 hours minimizes the radiation dose delivered to the patient. Nuclear medical imaging is unique in its ability to analyze organ structure and functioning. It allows the gathering of diagnostic information that would otherwise require surgery, or not be available at all. It also promotes early detection and treatment of cancers and other problems.
Two different technologies are used to provide the source of radiation for radioisotope production: nuclear reactors, which produce a flux of neutrons, and particle accelerators or cyclotrons, which produce a flux of charged particles, usually protons, but sometimes electrons or other particles. As an example, Tc-99m is the decay daughter product of Mo-99, an isotope with a half-life of 66 hours that is produced in nuclear reactors as a fission product of the neutron bombardment of a uranium target material. This is the source of all of the Tc-99m used in the world today. After irradiation, Mo-99 is recovered from the target, and packed into a production device from which technetium may be eluted in substantially pure form as needed for preparing radiopharmaceuticals for nuclear medical procedures
Other exemplary radioisotopes used for medical, industrial, research and commercial applications include thallium-201, which is used for medical cardiac imaging; calcium-44, which is used in bone growth studies; iridium-192, which is used for nondestructive testing of construction and other materials; cobalt-60, which is used to destroy cancer cells, to disinfect surgical equipment and medications, and the sterilization of food supplies; thulium-170, which is used for portable blood irradiations for leukemia, lymphoma treatment, and power source; gadolinium-153, which is used for osteoporosis detection and SPECT imaging; nickel-63, which can be used for the creation of long-life batteries; and americium-241, which is used in smoke detectors. In addition, rapid advances in nuclear medicine and other fields are focusing attention on a number of isotopes that have not previously been considered commercially important.
As an example of a conventional reactor used in radioisotope production, medical and industrial radioisotopes have been produced since 1957 by Canada's National Research Universal (NRU) reactor at the Atomic Energy of Canada's (AECL's) Chalk River Laboratories in Ontario, Canada. The NRU is a CANDU reactor designed for neutron beam research, materials research and medical/industrial isotope production. In general, CANDU plants are designed to use heavy water (deuterium oxide, or D2O) as the moderator and coolant. The use of heavy water, combined with other features of its design, allows the CANDU reactor to use natural uranium (NU) fuel, which is relatively inexpensive and abundant worldwide.
The NRU produces a high percentage of the world's medical and industrial radioisotopes, including molybdenum-99, a critical isotope used for medical diagnoses. In general, specimen rods containing an isotope target are inserted through penetrations in the NRU in a continuous process and subject to irradiation therein, so as to produce isotopes at a desired specific activity for use in nuclear medicine and/or industrial applications.
Two specialty reactors, the MAPLE 1 and MAPLE 2, are under construction at Chalk River Laboratories. These reactors are intended to replace the NRU. The MAPLE 1 and MAPLE 2 are dedicated exclusively for the production of medical radioisotopes. These research reactors are not intended nor designed for commercial power generation, since they are being designed for power levels of about 10 MWt. The MAPLE is a low-pressure, low-temperature, open-tank-in-pool type research reactor that uses low-enriched uranium (LEU) fuel. The core is compact, and is cooled and moderated by light water. Surrounding the light water core is a heavy water reflector tank, which maximizes the available neutron fluxes needed for radioisotope generation.